2.1. Liposomes
Liposomes are completely closed bilayer membranes containing an entrapped aqueous phase. Liposomes may be any variety of unilamellar vesicles (possessing a single membrane bilayer) or multilamellar vesicles (onion-like structures characterized by concentric membrane bilayers each separated from the next by a layer of water).
The original liposome preparations of Bangham et al. (1965, J. Mol. Biol. 13:238-252) involved suspending phospholipids in an organic solvent which was then evaporated to dryness leaving a waxy deposit of phospholipid on the reaction vessel. Then an appropriate amount of aqueous phase was added, the mixture was allowed to "swell", and the resulting liposomes which consisted of multilamellar vesicles (hereinafter referred to as MLVs) were dispersed by mechanical means. The structure of the resulting membrane bilayer is such that the hydrophobic (non-polar) "tails" of the lipid orient toward the center of the bilayer while the hydrophilic (polar) "heads" orient towards the aqueous phase. This technique provided the basis for the development of the small sonicated unilamellar vesicles (hereinafter referred to as SUVs) described by Papahadjapoulos and Miller (1967, Biochim. Biophys. Acta. 135:624-638). These "classical liposomes" (MLVs and SUVs), however, had a number of drawbacks not the least of which was a low entrapment of aqueous space markers.
Efforts to increase the entrapped volume involved first forming inverse micelles or liposome precursors, i.e., vesicles containing an aqueous phase surrounded by a mono-layer of lipid molecules oriented so that the polar head groups are directed towards the aqueous phase. Liposome precursors are formed by dispersing the aqueous solution to be entrapped in a solution of polar lipid in an organic solvent. The liposome precursors are then added to an aqueous medium and evaporated in the presence of excess lipid. The resultant liposomes, consisting of an aqueous phase entrapped by a single lipid bilayer are dispersed in aqueous phase (see U.S. Pat. No. 4,224,179 issued Sep. 23, 1980 to M. Schneider).
In another attempt to maximize the efficiency of entrapment Papahadjopoulos (U.S. Pat. No. 4,235,871 issued Nov. 25, 1980) describes a "reverse-phase evaporation process" for making unilamellar and oligolamellar lipid vesicles also known as reverse-phase evaporation vesicles (hereinafter referred to as REVs). According to this procedure, the aqueous material to be entrapped is added to a mixture of polar lipid in an organic solvent. Then a homogeneous water-in-oil type of emulsion is formed and the organic solvent is evaporated until a gel is formed. The gel is then converted to a suspension by dispersing the gel-like mixture in an aqueous media. The REVs produced consist mostly of unilamellar vesicles (large unilamellar vesicles or LUVs) and some oligolamellar vesicles which are characterized by only a few concentric bilayers with a large internal aqueous space. Certain permeability properties of REVs were reported to be similar to those of MLVs and SUVs (see Szoka and Papahadjopoulos, 1978, Proc. Natl. Acad. Sci. U.S.A. 75:4194-4198).
Liposomes which entrap a variety of compounds can be prepared, however, stability of the liposomes during storage is invariably limited. This loss in stability results in leakage of the entrapped aqueous soluble compound from the liposomes into the surrounding media, and can also result in contamination of the liposome contents by permeation of materials from the surrounding media into the liposome itself. As a result the storage life of traditional liposomes is very limited. Attempts to improve stability involved incorporating into the liposome membrane certain substances (hereinafter called "stabilizers") which affect the physical properties of the lipid bilayers (e.g., steroid groups). However, many of these substances are relatively expensive and the production of such liposomes is not cost-effective.
In addition to the storage problems of traditional liposomes a number of compounds cannot be incorporated into these vesicles. MLVs can only be prepared under conditions above the phase-transition temperature of the lipid membrane. This precludes the incorporation of heat labile molecules within liposomes that are composed of phospholipids which exhibit desirable properties but possess long and highly saturated side chains.